1. Field of the Invention
The present invention relates to an alloy type thermal fuse in which the operating temperature belongs to the range of about 120 to 150° C., and a wire member for such a thermal fuse element.
2. Description of the Prior Art
An alloy type thermal fuse is widely used as a thermo-protector for an electrical appliance or a circuit element, for example, a semiconductor device, a capacitor, or a resistor.
Such an alloy type thermal fuse has a configuration in which an alloy of a predetermined melting point is used as a fuse element, a flux is applied to the fuse element, and the flux-applied fuse element is sealed by an insulator.
The alloy type thermal fuse has the following operation mechanism.
The alloy type thermal fuse is disposed so as to thermally contact an electrical appliance or a circuit element which is to be protected. When the electrical appliance or the circuit element is caused to generate heat by any abnormality, the fuse element alloy of the thermal fuse is melted by the generated heat, and the molten alloy is divided and spheroidized because of the wettability with respect to a lead conductor or an electrode under the coexistence with the flux that has already melted. The power supply is finally interrupted as a result of advancement of the division and spheroidization. The temperature of the appliance is lowered by the power supply interruption, and the divided molten alloys are solidified, whereby the non-return cut-off operation is completed. Therefore, it is requested that the division temperature of the fuse element alloy is substantially equal to the allowable temperature of an electrical appliance or the like.
Usually, a low-melting alloy is used as such a fuse element. As apparent from a phase equilibrium diagram, an alloy has a solidus temperature and a liquidus temperature, and, at the eutectic point where the solidus temperature coincides with the liquidus temperature, the alloy is changed all at once from the solid phase to the liquid phase by heating which causes the alloy to pass the eutectic temperature. By contrast, in a composition other than the eutectic point, an alloy is changed in the sequence of the solid phase→the solid-liquid coexisting phase→the liquid phase, and the solid-liquid coexisting region temperature width ΔT exists between the solidus temperature Ts and the liquidus temperature T1. Even in the solid-liquid coexisting region, there is the possibility that the division of a fuse element occurs, although the possibility is low. In order to reduce the dispersion of the operating temperature among thermal fuses, it is requested to use an alloy composition in which the solid-liquid coexisting region temperature width ΔT is as narrow as possible. One of conditions imposed on an alloy type thermal fuse is that ΔT is narrow.
In many cases, a fuse element of an alloy type thermal fuse is used in the form of a linear piece. In order to reduce the size of a thermal fuse so as to comply with the recent tendency that appliances are further miniaturized, it is sometimes demanded to realize a thin fuse element. A fuse element is often requested to have drawability to a small diameter (for example, 400 μmφ or smaller).
In recent electrical appliances, the use of materials harmful to a living body, particularly metals such as Pb, Cd, Hg, and T1 is restricted because of increased awareness of environment conservation. Also a fuse element for a thermal fuse is requested not to contain such a harmful metal.
When alloy type thermal fuses are classified according to operating temperature, thermal fuses of an operating temperature of 120 to 150° C. are widely used.
As apparent from a phase equilibrium diagram of an In—Sn alloy, in an alloy of 85 to 52% In and a balance Sn, the liquidus temperature is 119 to 145° C. In this range, as compared with the range of an alloy composition of 52 to 43% In and a balance Sn where the liquidus temperature is similarly 119 to 145° C., the solidus temperature is higher, and hence the solid-liquid coexisting region temperature width is narrow. Therefore, the alloy of 85 to 52% In and a balance Sn satisfies the above-mentioned requirements such as that the reduced dispersion of the operating temperature, the operating temperature in the range of 120 to 150° C. (in a thermal fuse, usually, the fuse element temperature is assumed to be lower by several degrees centigrade than the surface temperature, and the operating temperature to be higher by several degrees centigrade than the melting point of the fuse element), and environment conservation of harmful metal free.
Usually, In has high ductility, and an alloy containing a large amount of In has excessive ductility, so that such an alloy is hardly drawn.
However, an alloy type thermal fuse of an operating temperature of 120 to 130° C. has been proposed in which, assuming that an In—Sn alloy containing In of 70% or less can be drawn, an alloy of 70 to 52% In and a balance Sn (the lower limit of In is set to 52% in order to suppress dispersion of the operating temperature as descried above) is used as a fuse element (Japanese Patent Application Laying-Open No. 2002-25402).
Because of load variations of an appliance, temperature variations, or the like, a thermal fuse is subjected to a heat cycle, and thermal stress is applied to a fuse element. In a usual alloy type thermal fuse, however, the characteristics of a fuse element is not changed by such thermal stress.
However, the inventors has noted that, when the above-mentioned In—Sn alloy containing In of 52% or more is used as a fuse element, a resistance variation of the resistance of a fuse element (rise of the resistance) is remarkably caused by a heat cycle. This phenomenon is produced by the fact that a slip in the interface between different phases in the alloy structure is increased, and such a slip repeatedly occurs, whereby a change of a sectional area or an elongation of the fuse element is caused in an excessive manner.
When such an increase of the resistance occurs, the temperature of the fuse element is raised by Joule's heat. Therefore, a problem is caused in which, when the temperature rise is indicated by ΔT, the fuse operates at a temperature that is lower than the allowable temperature of an appliance by the temperature rise ΔT, and, when the temperature rise ΔT is large, a serious operation error may occur.